1. Field of the Invention
The present invention relates to a hollow housing case for sheathing and electromagnetically shielding electric apparatus.
In general, a hollow housing case which can be fabricated easily is used in order to sheathe the body of an electric apparatus and to shield the electric apparatus against the influence of electromagnetic waves, electrostatic noise, the noise from a power supply, and the like.
2. Description of the Related Art
A hollow housing case is generally fabricated by the following procedures. First, a sheet of electrically conducting plate, such as metal plate, is deformed into a hollow form. Second, both ends of the hollow conducting plate are connected together electrically by means of spot welding or the like so that there is no electrical discontinuity in the hollow conducting plate. Third, the outer surface of the plate is sheathed by means of painting, coating with resin, or the like, in order to electrically protect the electric apparatus which are to be housed in the hollow case from external circumstances, and finally the fabrication of the hollow housing case is finished. Due to the above sheathing, the electric apparatus is securely insulated from the outside and also the outer surface of the hollow housing case looks attractive. On the other hand, it is necessary to electrically connect the inner surface of the hollow housing case to the body of the electric apparatus, in order to shield the apparatus. Therefore, during the process of the sheathing, the inner surface of the hollow case must be masked with tape or the like, so that painting material, resin, or the like do not adhere to the inner surface.
In order to avoid the problem of masking and reduce the time for fabrication of the hollow case, in the prior art, a conducting plate having one previously insulated surface outside and the opposite non-insulated surface inside has been prepared. In this case, however, both ends of the conducting plate cannot be connected together by means of spot welding or the like because the insulating surface at one end and the non-insulating surface at the other end contact with each other by simply overlapping both ends. Therefore, a piece of connecting plate to electrically connect together the non-insulating surfaces at both ends is arranged, so that one surface of the connecting plate contacts with both of the non-insulating surfaces. In this case, because the area of contact of the surfaces is relatively wide, it is difficult to connect together the non-insulating surfaces by means of spot welding. Accordingly, a piece of connecting plate and both ends of the conducting plate are connected together by pressing them at the same time with a press or the like. As mentioned above, in the prior art, the hollow housing case comprising the conducting plate which has one insulating surface and the opposite non-insulating surface is fabricated efficiently by means of pressing, without the process of masking.
However, the above pressing has the following disadvantage; namely, if the hollow case is relatively long along its axis, and similarly if the opening in the hollow case is relatively narrow in width, i.e., the length in the direction of pressing, it is difficult to fix the bed plate of the press in the desired position inside the hollow case. In this case, because the connecting plate and both ends of the conducting plate cannot be pressed with sufficiently large pressure, they might not be connected together securely. Consequently, the allowable dimensions of the hollow case in the process of pressing are limited. Further, because the press is generally expensive, the cost of fabrication may become rather high if a large number of the hollow cases are not to be fabricated.